1. Field of the Invention
The present invention relates generally to beam refraction systems, and more particularly to dynamic gain equalizers, spectrum analyzers and dynamic channel equalizers that are used in a dynamic compensation system that provides feedback across the entire spectrum.
2. Description of Related Art
Wavelength division multiplexing (WDM) provides a technique for increasing a transmission capacity by a single optical fiber. In a system adopting WDM, a plurality of optical carriers with different wavelengths are used. The plural optical carriers are individually modulated to obtain a plurality of optical signals. These are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light which is output to an optical fiber transmission line. On the receiving side, the WDM signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
In the case of incorporating an optical amplifier into a system adopting WDM, a transmission distance is limited by the wavelength characteristic of gain which is represented by a gain tilt or gain deviation. For example, in an EDFA, it is known that a gain tilt is produced at wavelengths in the vicinity of 1.55 mu m, and this gain tilt varies with total input power of signal light and pump light power to the EDFA.
A gain equalization method is known that measures against the wavelength characteristic of gain of an optical amplifier. A plurality of optical signals with different wavelengths are output from optical senders and are multiplexed in an optical multiplexer to obtain WDM signal light. The WDM signal light is then output to an optical transmission line which is configured by inserting a plurality of optical amplifiers, that compensate for losses, and at least one gain equalizer in an optical fiber transmission line. The WDM signal light transmitted by the optical-transmission line is separated into individual optical signals according to wavelengths by an optical demultiplexer. These optical signals are then supplied to optical receivers
If each optical amplifier has a wavelength characteristic of gain in the band of the WDM signal light, a gain tilt or gain deviation is accumulated over the length of the optical transmission line. This causes an interchannel deviation in signal power or signal-to-noise ratio. In the gain equalization method, the wavelength characteristic of loss of each gain equalizer is set in order to cancel the wavelength characteristic of total gain of the cascaded optical amplifiers. The wavelength characteristic of total gain is canceled by the wavelength characteristic of total loss in the band of the WDM signal light. In this manner gain equalization is achieved in the entire transmission line.
When an EDFA is used as each optical amplifier, the wavelength characteristic of gain of the EDFA is asymmetrical with respect to a wavelength axis in general. In contrast, the wavelength characteristic of loss of one optical filter usable as an element of each gain equalizer is symmetrical with respect to a wavelength axis in general. Accordingly, in the case that each gain equalizer includes only one optical filter, the asymmetrical wavelength characteristic of total gain of the cascaded optical amplifiers cannot be compensated. As the optical filter, a dielectric multilayer filter, etalon filter, Mach-Zehnder filter, etc. are known.
Gain equalizer is also achieved by combining two or more optical filters with different wavelength characteristics of loss. With this configuration, the wavelength characteristic of gain can be canceled by the wavelength characteristic of loss with high accuracy in a given band of WDM signal light.
Additional information on gain equalization methods is described in N. S. Bergano et al., “Wavelength division multiplexing in long-haul transmission systems”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 14, NO. 6, JUNE 1996, pp 1229-1308. (2) K. Oda et al., “128-channel, 480-km FSK-DD transmission experiment using 0.98 mu m pumped erbium doped fibre amplifiers and a tunable gain equaliser”, ELECTRONICS LETTERS, 9 Jun. 1994, Vol. 30, No. 12, pp 982-983. (3) T. Naito et al., “85-Gb/s WDM transmission experiment over 7931-km using gain equalization to compensate for asymmetry in EDFA gain characteristics”, First Optoelectronics and Communications Conference (OECC '96) Technical Digest, July 1996, PD1-2. (4) T. Oguma et al., “Optical gain equalizer for optical fiber amplifier”, Communications Society Conference, IEICE, 1996, B-1093 (pp 578).
The wavelength characteristic of gain of an optical amplifier changes according to operating conditions such as a pumped condition of the optical amplifier and an input power of signal light. In a submarine optical repeater system the input power to an optical amplifier may change because of an increase in optical fiber loss due to aging or because of cable patching for repairing. Such a change in system condition causes a change in operating conditions of the optical amplifier, resulting in a change in its wavelength characteristic of gain. Further, there is a possibility that the wavelength characteristic of gain may deviate from a design value because of variations in quality of optical amplifiers manufactured.
In the conventional gain equalization method using an optical filter with a fixed wavelength characteristic of loss, problems occur when the wavelength characteristic of gain of an optical amplifier changes because of changes in system conditions. In this instance, the new wavelength characteristic of gain of the optical amplifier does not coincide with the wavelength characteristic of loss of the optical filter. This produces an equalization error. The equalization error varies according to the system condition, and a large amount of variations in the equalization error may cause an interchannel deviation in signal power or optical SNR or may remarkably deteriorate a transmission quality in a certain channel. However, the conventional variable gain equalizer cannot obtain an arbitrary wavelength characteristic of loss in response to variations in equalization error, so that variations in equalization error due to changes in system condition cannot be sufficiently suppressed.
There is a need for a dynamic compensation system that provides feedback acros the entire spectrum.